A conventional printed circuit board has a plurality of heat-producing components (e.g., semiconductors) mounted on it. These heat-producing components are specified with their permissible operating temperatures, but the actual operating temperatures often exceed the permissible temperatures due to their own heat and thermal influence of the adjacent heat-producing components. It is necessary for this reason to dissipate heat of the heat-producing components using a heat radiation device. One example of such device is a heat sink 603 of a shape having a plurality of heat dissipating fins made by extrusion of a metal material such as aluminum, which is placed in contact with heat-producing components 602a and 602b to radiate the heat of heat-producing components 602a and 602b, as shown in FIG. 8.
FIG. 8 is a perspective view showing the structure of the conventional heat radiation device, in which heat-producing components 602a and 602b such as transistors of a power amplifier circuit are fixed with screws to heat sink 603 serving as the heat radiation device through an insulating material (not shown). Heat sink 603 is fixed to a chassis (not shown) disposed within enclosure 601.
Heat sink 603 is normally casted with aluminum or the like material, and provided with a plurality of fins 603a. The heat generated by heat-producing components 602a and 602b heats up heat sink 603, which produces the phenomenon of natural convection through a ventilator (not shown) or vent openings 604a and 604b formed in bottom plate 601a and top plate 601b of enclosure 601 where heat sink 603 is mounted, and introduces outside air from vent opening 604a in bottom plate 601a as indicated by an arrow. In this conventional heat radiation device, the heat generated by heat-producing components 602a and 602b flows out of vent opening 604b in top plate 601b as indicated by another arrow. As illustrated, the conventional heat radiation device uses a natural air-cooling method.
This kind of conventional heat radiation method is effective when power consumption of heat-producing components 602a and 602b is small, and there are no other heat-producing components in the vicinity of them (refer to patent literature 1 for example).
On the other hand, printed circuit boards keep following the trail of downsizing in addition to continuous increase in power consumption of the heat-producing components (e.g., semiconductors) in line with advancement of their functions. For this reason, heat-producing components are disposed as close as several millimeters to one another on a printed circuit board. As a result, temperature of certain heat-producing components rises above their permissible operating temperatures due to thermal influences of other heat-producing components located in the vicinity thereof and operating at higher temperatures by several tens of degrees Celsius.
Traditionally, this problem has been dealt with by providing a heat sink in contact only with the heat-producing components whose operating temperature exceed their permissible operating temperatures, and lowering the operating temperatures of these heat-producing components. However, due to a closely arranged condition of heat-producing components, it may become necessary to take additional measures. For example, to lower the ambient temperature of the heat-producing components having temperature rise above their permissible operating temperatures, at the same time it may become necessary to dissipate the heat of other adjacent heat-producing components whose operating temperatures are higher by several tens of degrees Celsius.
It is desirable in this case to fix one each of heat sinks to the individual heat-producing components. However, it becomes difficult to mount such heat sinks that have sufficient heat radiating areas required for cooling since the heat-producing components are placed close to one another. A method conceivable to cool the plurality of heat-producing components is to use heat sink 719 of a single-piece structure that is large enough to cover the plurality of heat-producing components 713 and 714, which are the targets of heat dissipation, and mount the heat sink 719 in a position astride both of heat-producing components 713 and 714 as shown in FIGS. 9A and 9B.
In FIGS. 9A and 9B, heat-producing component (e.g., a semiconductor) 714 is assumed to have a safety margin of only several degrees Celsius against the permissible operating temperature. As a result, it is necessary to decrease the ambient temperature by lowering operating temperature of adjacent heat-producing component (e.g., a semiconductor) 713 in addition to dissipating heat of heat-producing component 714 in order to keep the operating temperature within the tolerable level. As shown in FIGS. 9A and 9B, the method is to make heat-producing components 713 and 714 in contact with heat sink 719 of aluminum in a shape including a plurality of fins, through heat conductive rubber 720 to dissipate the heat. Aluminum generally has a thermal conductivity of about 200 to 300 W/(m·K).
Distribution of heat in conventional heat sink 719 of this structure is shown in FIG. 10, which illustrates heat sink 719, heat-producing components 713 and 714, other components 730 and 731 on printed circuit board 711, and temperature measurement points a2, b2, c2 and d2. FIG. 10 also shows temperature values measured at individual temperature measurement points a2, b2, c2 and d2. As shown in FIG. 10, the heat is distributed generally uniformly throughout conventional heat sink 719, as the temperatures are about 57° C. throughout all the measurement points a2, b2, c2 and d2. However, heat-producing component 714 which has the margin of only several degrees Celsius against the permissible operating temperature may breakdown since the operating temperature of 57° C. exceeds the permissible temperature of 55° C. of the heat-producing component 714.
To this end, the conventional method of dissipating the heat from heat-producing components 713 and 714 presents a problem in that the temperature of heat sink 719 exceeds the permissible operating temperature of heat-producing component 714 having the margin of only several degrees C. against the permissible operating temperature when both of heat-producing components 713 and 714 of different permissible operating temperatures and power consumptions are cooled with single heat sink 719.
Although it is conceivable to increase a surface area of heat sink 719 or to extend a height of the fins to bring down the temperature of heat-producing component 714 to the permissible operating temperature or below, this is difficult due to design requirements of printed circuit board 711.